Centrifugal fan impellers have blades of equal chord length. This ensures that the impeller will be rotationally mass-balanced by design, and, in the absence of a fan housing or scroll, also ensures that the velocity profile and flow rate of the air leaving each blade passage will be equal. Further, the airflow caused by the blades may include an acoustic output at a frequency related to the rotational speed of the blades.
In some cases, when a fan housing covers the blades, the fan housing may include a protruding region, or “throat,” that forms the beginning of the scroll of increasing radius responsible for pressure development in the fan. This throat is separated by a relatively close distance to the blades, or “throat gap.” The throat and throat gap are designed in part to redirect part of the airflow away from the tangential/rotational direction and towards a direction normal to the fan outlet plane to the airflow efficiency of the fan. However, a significant jet of air is likely to impinge on and around the throat, thereby increasing static and fluctuating pressures and, hence, the amplitude of the acoustic output. This may cause the fan to generate increased acoustic noise. Further, when the described fan and fan housing are installed in an electronic device, the increased acoustic noise leads to the electronic device producing unwanted noise.